RF Technologies for Low Power Wireless Communications

Several years ago a program in low power electronics initiated by the Army Research Office focused on addressing the issue of RF and microwave systems with a major concern for the prime power required for wireless transmitters. About the same time, DARPA initiated a program to address the reduction of power in digital and computing systems. As a complement to these programs, the Office of the Secretary of Defense initiated a multidisciplinary university research initiative (MURI) program to augment the Army program in RF and microwave systems.

"The goal of this research program was to develop techniques to accomplish the RF function at the lowest expenditure of energy."

This program ran for five years and involved researchers in four universities: the University of California at Los Angeles (UCLA), the University of California, San Diego (UCSD), the University of Michigan and the University of Colorado at Boulder. The principal investigators from that MURI are the authors of this book, which presents the results of the sponsored research. The material in the book is presented to the design community in order to take advantage of the research in reducing power consumption in RF systems. The MURI effort focused primarily on communications systems. However, most of the research concepts can be applied to other RF systems, such as radar or target seekers in missiles.

The goal of this research program was to develop techniques to accomplish the RF function at the lowest expenditure of energy. The resulting comprehensive approach requires a highly interdisciplinary effort involving device and semiconductor science, circuit engineering, electromagnetics, antenna engineering and communications systems engineering. It is not possible in this book to go into as much technical detail in each topical area as is contained in the many technical publications resulting from the research. The book attempts to present the concepts and conclusions in an understandable manner and to allow the reader to reference the detailed publications for more in-depth information, as required.

By the author's own admission, this book is a sequel to his previous publication RF Power Amplifiers for Wireless Communications (Artech House Inc., 1999). It offers a more in-depth understanding of the theory of RF power amplifiers and covers some topics that were barely mentioned previously, such as bipolar junction transistors (BJT) and others that deserve a more detailed treatment.

With the advent of heterojunction bipolar transistor (HBT) and silicon germanium (SiGe) technology, interest in these devices has increased. Chapter 1 revisits the basics of class AB operation with higher emphasis on BJT applications. Chapter 2 is devoted to the Doherty and Chireix techniques, which still appear to be rooted in history and vacuum tubes. This chapter addresses their potential role in modern applications. This includes a more generalized analysis of the Doherty PA and some simulation results on practical implementations.

"With the advent of heterojunction bipolar transistor (HBT) and silicon germanium (SiGe) technology, interest in these devices has increased."

The Chireix method continues to be a talking point and not much else. The chapter attempts to explain why it should be so. Chapter 3 takes up the theme of nonlinear effects in PAs, with particular emphasis on the problem of asymmetrical IM distortion, its causes and possible remedies. The chapter also has some tutorial material on the behavioral modeling of PAs and attempts to show that there is still much useful life left in polynomial and Volterra series modeling methods. Chapters 4, 5 and 6 cover the three main linearization topics of feedback, pre-distortion and feed-forward. Microwave power amplifier applications also get a chapter of their own, Chapter 7, in which an attempt is made to pull together some of the techniques which were developed in the 1970s and 80s for higher frequencies (> 2 GHz) broadband amplifier design and which could easily come back on stream again as wireless communications run out of bandwidth at the low end of the microwave spectrum.